1. Field
Embodiments of the present invention relate to a laser welding machine and a laser welding method for joining a lead frame having a vertically separated construction in a process of assembling a power semiconductor module.
2. Description of Related Art
A large current carrying capacity as well as a small size are required recently by power semiconductor modules composing an inverter circuit used in motor control in electric vehicles and in elevator control. This causes high current density and large heat generation in power semiconductor chips such as insulated gate bipolar transistors (IGBTs) installed in power semiconductor modules. As a consequence, heat dissipation design has to be reviewed for conventional semiconductor modules with aluminum wire-connected construction.
Patent Document 1 discloses a semiconductor module installing an IGBT chip as shown in FIG. 4, which is a sectional view of a basic construction part of this conventional semiconductor module. This semiconductor module comprises a copper base 200, which is a heat sink with a rectangular shape, an insulating substrate 202 with a rectangular shape joined through a solder 201 onto the copper base 200, and an IGBT chip 204 joined onto the insulating substrate 202. The upper periphery 203 of the copper base 200 is fitted and adhered to the lower inside part of a resin case 205 surrounding the IGBT chip 204 and the insulating substrate 202. Externally leading out terminals of an emitter terminal 206 and a collector terminal 207 are integrated monolithically with the resin case 205 on the side wall surface of the resin case 205 opposing each other in the resin case 205. The emitter terminal 206 and the collector terminal 207 are electrically connected through aluminum wires 208 and 209 to circuit patterns 210 and 214 formed on the surface of the insulating substrate 202, although the circuit pattern is not clearly depicted in FIG. 4. The circuit pattern 210 is connected through an aluminum wire 213 to an emitter electrode on the upper surface of the IGBT chip 204. A collector electrode on the back surface of the chip is electrically connected to the other circuit pattern 214. The emitter electrode and the collector electrode are not depicted in FIG. 4.
More specifically, the emitter electrode on the upper surface of the IGBT chip 204 is joined through solder 211 to a heat spreader 212. The heat spreader 212 is a metallic plate provided to enhance dissipation of heat generated by current conduction in the IGBT chip 204 and is electrically connected to the emitter electrode formed on the upper surface of the IGBT chip. To the upper surface of the heat spreader 212 connected by bonding is an end of the aluminum wire 213 mentioned previously, while the other end of the aluminum wire is connected by bonding to the circuit pattern 210 formed on the insulating substrate 202. Large current carrying capacity can be achieved by this construction of aluminum wire bonding connection to the chip upper surface through a heat spreader as disclosed in Patent Document 1.
Patent Document 2 discloses another means for improving heat dissipation from the upper surface of a semiconductor chip, which uses a structure of surface terminal plate of a lead frame composed of modified cross-sectional plates with different thicknesses made of a high thermal conductivity copper plate as shown in FIG. 5 in place of aluminum wires connecting to the chip surface.
Patent Document 3 discloses a method to obtain a necessary terminal structure that uses a plurality of lead frames of copper in place of aluminum wires, on a heat spreader, a heat radiating plate, soldered on a semiconductor chip, and the plurality of lead frames are joined by laser welding.
Patent Document 4 discloses a structure in which a lead frame is divided into two parts: an electrode plate working as a heat spreader, and a wiring member, and after brazing the electrode plate onto the surface of a chip with a hard solder, the joining part of the wiring member is overlapped with the extension of the electrode plate, and then the two plates are joined by laser welding.
Patent Document 5 discloses a method of controlling a focal distance of a laser welding machine.
Patent Document 6 discloses a method of measuring and controlling a focal distance of a laser welding machine using a laser displacement meter or a contact type displacement meter,